This invention relates to titanium-containing catalysts for producing olefin polymers.
It has long been known to produce olefin polymers utilizing a supported chromium catalyst as disclosed in Hogan et al U.S. Pat. No. 2,825,721. More recently it has been discovered that certain advantages can flow from the presence of a titanium compound in the catalyst as shown, for instance, by Nasser, Jr. U.S. Pat. No. 3,798,202.
It is well known that polymer properties can be influenced by variations in the process conditions and by variations in catalyst composition. For instance operation at a lower reactor temperature tends to give lower melt index (higher molecular weight) polymer and the reaction rate may be slowed down. As shown by said Nasser, Jr. patent, it is known that the melt index can be increased by the incorporation of a titanium compound. As a general proposition lower melt index, broader molecular weight distribution (high HLMI/ML ratio) polymers of similar densities tend to have better resistance to environmental stress cracking. Also the incorporation of comonomer to lower the density also can tend to increase environmental stress crack resistance with polymers having similar melt indexes. However, factors which bring about improvement in stress crack resistance generally result in a compromise in other properties. For instance lowering the melt index will result in a product which is less processable and which has reduced stiffness. Thus frequently a sacrifice must be made in one property if an improvement is to be made in another.